Birds Calling in Dark Woods: Bioacoustic Avian Population Surveys in the Low-light Conditions of Northern German Woodlands throughout the Winter Months
Acoustic monitoring of birds has become increasingly important in recent years, particularly for surveys in inaccessible or structurally complex terrain. Most recently, it has also emerged as a reliable method in standardized breeding bird surveys, which is why the Planungsgruppe Grün GmbH, located in Oldenburg, Germany, decided to use it for their comprehensive surveys of breeding bird populations in northern Germany.
Fig. 1: A BAR-HC mounted on a tree in a dense coniferous forest. In the winter months, the sun rises low above the horizon, resulting in very short daylight hours in northern European regions. Photo Credit: Planungsgruppe Grün GmbH |
To carry out this work, the team needed a device that could record reliably in the sub-zero, humid, and dark environment of northern German woodlands and swamps, while needing minimal servicing over a 6-month-period. Their choice fell on Frontier Labs Solar BARs, as they seemed suitable for their purposes. However, because the recorders were to be deployed in dense forests with limited sunlight reaching the forest floor, and because early spring in the region is often overcast with short daylight hours, a modified version of the Solar BAR, the BAR-HC, was offered to them. This recorder comes without the solar panel, but sports a large-capacity lithium-ion battery instead, advertised to offer 18000 hours of recording on a single charge. This attribute makes the BAR-HC a great choice for long-term studies in dark environments, like dense forests, caves, or regions with very short winter daylight hours.
Fig. 2 and 3: A direct comparison of the BAR-HC (left) and the Solar BAR (right). Clearly noticable is the lack of a solar panel on the BAR-HC, making it perfect for long-term deployments in low-light environments. Just like the Solar BAR, it can be deployed on a starpicket, or strapped directly to a structure.
Mounting the devices on the selected trees was straightforward using the supplied ratchet straps and mounting plate. None of the recorders shifted during the entire deployment period. The BAR-HCs were in use from early March to mid-August.
Midway through the survey season, an SD card change was carried out to back up the valuable data, and to allow responses to potential rare species during targeted on-site checks. This process was completed without any data loss.
Using the manufacturer’s software, various recording schedules were easily configured, allowing more than eight hours of data to be recorded daily throughout the study period. All devices adhered reliably to this schedule, resulting in complete coverage. Recording start times could be defined either as fixed clock times or dynamically adjusted to sunrise and sunset throughout the year. Time synchronization via the GPS modules worked precisely for the entire duration of the survey, and protocol entries were generated reliably for all recordings. Despite operating on a continuous eight-hour-per-day recording schedule for six months, by the end of the survey period, well over half of the battery capacity remained on each of the devices.
With the help of the BAR-HCs, species were detected in the study area that would otherwise have been discovered only with great luck through costly, personnel-based surveys due to their brief presence in the study area. In addition, the devices allowed for long-term, disturbance-free monitoring of breeding success at a raptor nest.
Overall, the Planungsgruppe Grün GmbH is very satisfied with the deployment and looks forward to implementing a new project this year in a poorly accessable swamp area.
With so many applications for bioacoustic technology, we're constantly inspired by our customers' projects and their findings.
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